FEATURE ARTICLE: PRELIMINARY REBASING OF AUSTRALIA'S POPULATION ESTIMATES USING THE 2011 CENSUS OF POPULATION AND HOUSING

INTRODUCTION

In this issue, the ABS has used the 2011 Census results to undertake two main sets of calculations to produce preliminary rebased population estimates. These estimates will be updated again to produce final rebased estimates in the December quarter 2012 issue of this publication (released in June 2013).

The first set of calculations was applied to construct a new preliminary estimated resident population (ERP) for 30 June 2011 from which to then estimate quarterly ERP forward. This was done to ensure that population estimates for the next intercensal period (i.e. 2011-2016) are as accurate as possible. Because this new population estimate uses the Census as its main data source, it is said to be 'based' on the 2011 Census and is referred to as a population base.

The second set of calculations was made to revise the 19 intercensal quarterly estimates preceding June quarter 2011 (i.e. September quarter 2006-March quarter 2011) to produce 'preliminary rebased' estimates. When the status of these estimates is changed to 'final rebased' in the December 2012 issue of this publication (to be released in June 2013), no subsequent revisions will be made to these estimates. Making this adjustment ensures that the ERP time series for the previous intercensal period are comparable with the latest estimates. The following provides a more detailed explanation of the adjustments made using the 2011 Census results and some of the related rebasing concepts.

CONSTRUCTING THE PRELIMINARY ERP BASE FIGURE FOR 30 JUNE 2011

Constructing a preliminary 30 June 2011 ERP figure from the 2011 Census count involves three main steps. The first two relate to determining the ERP at census night (9 August 2011) and the third backdates ERP to 30 June 2011.

The first step addresses the retabulating of Census counts by actual location to reflect a Census count by place of usual residence by:

adding in an estimate of those residents who were absent interstate on Census night;

subtracting those who were visiting from interstate; and

subtracting overseas visitors to Australia on Census night;

The second step addresses those who were missed in the census by:

using the results from the 2011 Census Post Enumeration Survey (PES) to adjust for undercount;

adding in the number of Australian residents who were temporarily overseas (RTOs) on Census night using data on international travellers obtained from the Department of Immigration and Citizenship; and

applying a range of demographic adjustments designed to resolve any statistical anomalies in the age sex composition of the derived population estimates.

The third step addresses backdating the resulting ERP figure for Census night 9 August 2011 to 30 June 2011 using the components of population change by:

subtracting births;

adding in deaths;

accounting for net interstate migration; and

accounting for net overseas migration

These steps are illustrated in the following flow chart:

Census to ERP process

The table below shows how the ABS arrived at the Census night 9 August 2011 ERP figure for Australia, states and territories using figures obtained from applying the adjustments listed above.

(a) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

(b) Includes Census net undercount from the 2011 PES and minor adjustments to address additional data coherence and quality matters.

(c) These estimates are preliminary and may change in compilation of final rebased estimates.

(d) Component data calculated for the period 1 July to 9 August 2011.

INTERCENSAL ERROR AND INTERCENSAL DISCREPANCY

The intercensal error refers to the difference between the latest Census based ERP figures for 30 June 2011 and the ERP figures based on the previous Census which have been carried forward using births, deaths and migration data.

There are two areas that contribute to the intercensal error:

errors in the Census based estimates of the population at the current or previous Census date; and/or

errors in the estimates of any of the components of population change (births, deaths and migration) since the previous Census.

The table below shows the preliminary intercensal error by number and percentage of total population for the 2006-2011 period for Australia, states and territories.

Intercensal Error(a), 2006 - 2011

Intersensal Error

Intersensal Error

'000

%

New South Wales

90.7

1.3

Victoria

87.0

1.6

Queensland

106.2

2.4

South Australia

18.1

1.1

Western Australia

-2.9

-0.1

Tasmania

-0.7

-0.1

Northern Territory

-1.0

-0.4

Australian Capital Territory

-2.1

-0.6

Australia(b)

294.4

1.3

(a) A positive number indicates that unrebased ERP as at 30 June 2011 was higher than rebased ERP. A negative number indicates it was lower than rebased ERP.

(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

Information collected in the 2011 Census will also allow the ABS to estimate approximately how much of the intercensal error is due to inaccuracies in estimates of interstate migration. In order to do this, the ABS will analyse data from the Census questions concerning an individual's place of usual residence one year ago and five years ago. These results will be released in the December 2012 issue of this publication (released in June 2013).

After the intercensal error is adjusted for revisions to the components of population change (births, deaths and migration), the remaining (unattributable) portion is referred to as the intercensal discrepancy. The estimate of intercensal discrepancy for each state and territory, birth cohort and sex are spread evenly across the intercensal quarters. Thus the intercensal discrepancy acts as a balancing item, that when combined with births, deaths and migration equals the difference between the two Census population estimates. Intercensal discrepancy is caused by errors in the start and/or finish population estimates and/or in estimates of births, deaths or migration in the intervening period which cannot be attributed to a particular source. The 2006-2011 intercensal discrepancy will be published in the December 2012 issue of this publication, to be released in June 2013.

REVISING THE 20 MOST RECENT QUARTERLY ESTIMATES TO 'PRELIMINARY REBASED'

The main purpose of revising the 20 most recent intercensal estimates (i.e. September 2006-June 2011) of quarterly population growth to 'preliminary rebased' was to ensure that the estimates from the 2006-2011 intercensal period will be comparable to all future estimates, thus creating a consistent time series of ERP data. These estimates will remain as 'preliminary rebased' until the December 2012 issue of this publication (released in June 2013) when their status will be changed for the last time to 'final rebased'. Following this, no subsequent revisions will be made to these estimates.

ADJUSTING FOR NET UNDERCOUNT

Net undercount for Australia in the 2011 Census was 374,600 persons (including additional minor coherence and quality adjustments). Net undercount is the difference between the actual Census count and the estimate of the number of people who should have been counted in the Census. This estimate is based on the PES conducted in August and September of 2011. For a category of person (based on age, sex and state of usual residence), net undercount is the resultant of Census undercount, overcount, misclassification and imputation error. Adding the net undercount of people back into the population is a crucial step in arriving at the most accurate ERP possible. For more information on measuring net undercount using the PES see Information Paper: Measuring Net Undercount in the 2011 Population Census, 2011 (cat. no. 2940.0.55.001) and Census of Population and Housing - Details of Undercount, 2011 (ABS cat. no. 2940.0).

PES IMPROVEMENTS

The 2011 PES involved a considerable degree of innovation, with the most important change being the introduction of Automated Data Linking (ADL). This new methodology, which was tested in the 2006 PES employed probabilistic linking techniques, using a range of personal and address characteristics, to evaluate the likelihood that a PES and Census record pertained to the same individual. ADL therefore provided the opportunity to match persons who would have been too difficult to match previously, given the constraints of previous technology and processes.

Other notable improvements include:

Minimal overlap between Census post-collection follow up and the start of PES enumeration;

Changes to the PES questionnaire to improve the collection of Indigenous status;

Doubling the sample of Aboriginal and Torres Strait Islander people, compared to 2006, as a result of focused sampling in areas within the 'mainstream' sample with a high density of Aboriginal and Torres Strait Islander people and an increase in the sample from discrete Indigenous communities; and

Following this issue which contains preliminary population estimates for Australia, states and territories based on the 2011 Census, the ABS will also publish final population estimates based on the 2011 Census for Australia, states and territories, for September quarter 2006 to June quarter 2011, in the December quarter 2012 issue of Australian Demographic Statistics (cat. no. 3101.0) to be released on 20 June 2013. That issue will include final rebased estimates dating back to 30 September 2006. No subsequent revisions will be made to those final rebased estimates.

Preliminary rebased estimates for SA2s and LGAs will be published in Regional Population Growth, Australia, 2011 (cat. no. 3218.0) with final rebased estimates published by mid August 2013. Preliminary rebased estimates of the Australian Aboriginal and Torres Strait Islander population at 30 June 2011 will be published by five year age groups, sex and state/territory in the next issue of this publication on 27 September 2012. No further disaggregation will be released until final estimates are compiled and released by mid August 2013.